The invention relates to the field of electro-optic modulators, and in particular to a novel high-dynamic-range ultra-wideband (UWB) chip-scale modulator with built-in tunable microsecond true time delay.
Current state-of-the-art chip-scale analog modulator technology generally provides negligible true time delay, and thus dedicated analog delay lines must be employed. However, there is a lack of compact tunable delay lines at microsecond level while being tunable, in terms of amplitude, phase and delay, and also being integratable in array forms on a semiconductor chip. For example, to cancel the echo effects from environmental scatterers, the analog cancellation filter in full-duplex transceivers demand larger numbers of taps in FIR configurations. And each tap ideally can be dynamically adjusted in amplitude and phase to accommodate the change in environment and the different echo cross-section of various scatterers. However, the dimension of a single-tap FIR SiC filter in current full-duplex systems has already been on the order of tens of centimeters (microwave delay lines) for merely few ns delay. Integrated RF-photonics can pack such centimeter-long delay line on a millimeter-scale chip, but only allows fixed delay. Surface-acoustic wave technology has similar limitations. In what follows, the three main delay line technologies are compared and discuss their fundamental limitations.
The simplest form of an analog RF delay line is a RF or microwave transmission line. For example, micro-strips on a PCB can provide delay up to a few nanosecond, and have already been used for self-cancellation in full-duplex radios. Packed coaxial cables are also commercially available for longer delays up to a few hundreds of nanosecond. However, several major issues have plagued RF transmission line, which in turn triggered the birth of analog RF photonics in 1980s. RF transmission lines produce 3.3-μs delay per kilometer of length: hundreds of meters of cable are required for μs-level delay (typical for self-cancellation application), which are typically bulky and too costly in practice. RF transmission lines also exhibit higher losses at higher frequencies, due to the skin-effect and proximity effect, limiting the practical use of such RF delay lines to frequencies below 1 GHz. In addition, the phase response of RF cables are vibration-sensitive, and most RF transmission lines are susceptible to electromagnetic interference. Adding to these difficulties, adjustable delay is costly to realize due to the size and power required by RF switches.
Alternatively, an analog delay line can be realized using RF-photonics, which up-converts RF signal to an optical carrier, uses optical fibers or on-chip optical waveguides to realize the delay in the optical domain, and later down-converts the optical signal to RF domain. A typical system consists of three components: a modulator (electrical-to-optical conversion), an optical delay line (a waveguide or free-space propagation region), and a photodetector (optical-to-electrical conversion). For most RF-photonic systems, the property of the optical link dictates the RF transfer function, and the RF-photonic delay line produces a true-time-delay between the RF input and output, which is roughly equal to the photon transit time over the length of the optical delay line. The transit time amounts to 4.90 μs per kilometer of optical fibers in conventional discrete systems and 10 μs per kilometer of silicon waveguides in on-chip integrated systems.
Clearly, packing μs-level delays on a millimeter-sized chip still represents a major challenge: one needs to accommodate hundreds of meters of waveguides and more than 10 dB/μs on-chip propagation loss even for the ultralow-loss TriPleX waveguides. Major advantages of RF-photonic delay lines are well known. These include wide bandwidth, frequency-independent low propagation loss, immunity to electromagnetic interference, frequency tunability, compact size and robustness (against RF cable delay lines), and high spurious-free dynamic range (SFDR). The SFDR is governed by the third-order nonlinear response of the modulator (a Mach-Zander modulator for GHz applications), and SFDR as high as 130 dB·Hz^(⅔) has been demonstrated with noise figures below 10 dB. One major issue for RF-photonic delay lines is the prohibitive cost, size and complexity associated with realizing multiple, distinct and adjustable delays, each with nanosecond resolution as needed by the SiC applications. State-of-the-art discrete systems realized with fiber-optic switches can provide step-wise tuning at 1 μs resolution, however each tap of an FIR filter would requires a rack-mount enclosure of 21-inch across to accommodate such a system. Additionally, fiber-based RF-photonic delay lines suffer from polarization noise caused by external vibrations.
Another approach to chip-scale analog delay is provided by acoustic delay lines, which are realized through reversible piezoelectric conversion between microwaves and acoustic waves. Such an acoustic delay line is made of two piezoelectric interdigitated transducers (IDTs) at a fixed distance. Either bulk acoustic waves (10-200 MHz) or surface acoustic waves (up to 2.5 GHz) are excited by one transducer from the input RF signal, traverse the device, and are converted back to a RF signal by the 2nd transducer. One of the biggest benefit of acoustic delay lines is their compact size. Because the sound velocity is in the range between 3000 and 5000 m/s in typical single-crystalline substrates, about 100,000× slower than that of microwave or light, a 1-mm long chip can produce similar levels of delay to a 100-meter long cable or fiber, essentially 100,000× smaller than a RF or photonic delay line.
However, acoustic delay lines have many significant challenges. First, the distance between piezoelectric transducers are fixed by lithography, thus the resultant acoustic delay is determined by the device fabrication (i.e., not readily tunable). Tunable acoustic delay typically requires a highly dispersive delay line and frequency up-conversion and down-conversion, which is inherently narrow-band, with limited tuning range, and noisy. Second, acoustic delay lines have limited operation frequency range, since the efficiency of piezoelectric SAW transducer drops significantly beyond 3 GHz.
Third, electromagnetic feedthrough represents a fundamental challenge, when SAW delay lines are deployed in SiC filters. As one of the biggest parasitic effect, electromagnetic feedthrough in SAW delay lines originates from the direct RF pickup between the electrodes of the input and output IDTs.
This interference produces periodic ripples in amplitude and phase responses across the passband of the delay line. Around 2.4 GHz, the strength of the electromagnetic feedthrough is typically greater than −50 dB of the input level, and increases with frequency as the capacitive reactance decreases. Particularly, in multi-tap filters, built from multiple fixed delay lines, the electromagnetic feedthrough creates an additional self-interference pathway that greatly exceeds the dynamic range of the digital cancellation filter. Conventional SAW delay lines also exhibit significant durability issues for high-power RF inputs. Typical power durability of a SAW device is about 15 mW for 100,000 hours, due to the thermal stress produced by the high electrical resistance of the electrodes and the high dynamic stress between the narrow IDT fingers from the stress profile of the surface acoustic waves. These two problems are much less severe in bulk acoustic wave (BAW) devices, which can have roughly 50× higher power durability. However, BAW delay lines are limited to operate below 1 GHz.
All these challenges make it important to seek novel physical mechanisms in analog cancellation, to provide scalability in carrier frequencies, compact size, power efficiency, and ultra-broad bandwidth.